1. Field of the Invention
The present invention relates in general to a power supply apparatus for computers, monitors, and other computer peripheral devices. In particular, the present invention relates to a power supply apparatus for starting a monitor according to the power signal supplied from the computer host. When the computer is in the Display Power Management Signaling mode, as DPMS hereinafter, the power supply apparatus is disabled to turn off the monitor and stops supplying the power source to the elements in the power supply apparatus for economizing on the power in the computer system.
2. Description of the Related Art
Recently, switching type power supply apparatuses have been used in computers and computer peripheral devices. In general, a pulse-width modulation controller (PWM controller) is utilized as a power supply control circuit for a switching type power supply. The PWM controllers such as the 3842 IC and 3844 IC series require sufficient voltage and current to start their control functions normally, so a starting circuit is necessary. However, once the PWM controller has started, the energy for activating the PWM controller is supplied by an auxiliary power source. Therefore, the starting circuit becomes a redundant circuit and continuously consumes power.
FIG. 1 shows a circuit diagram of a conventional switching type power supply apparatus, and a stabilizing control circuit for stabilizing the output voltage is omitted.
As shown in FIG. 1, a rectifier 1, such as a full-bridge rectifier, rectifies an alternating-current (AC) voltage into a direct current (DC) voltage, and a filter capacitor 2 filters the rectified voltage and reduces the voltage ripple. The rectified and filtered voltage charges a starting capacitor 4 through a starting resistor 3 and thus the starting resistor 3 supplies the rectified and filtered voltage to a power supply control circuit 5 (here, the power supply control circuit is a 3842 controller, for example). After activating the power supply control circuit 5, the power supply control circuit 5 issues a switching signal to a control transistor 6, such as a NMOS transistor. The gate of the NMOS transistor 6 is connected to the power supply control circuit 5.
A power transformer 7 has a primary winding 7a, a secondary winding 7b and a tertiary winding 7c.The primary winding 7a of the power transformer 7 is connected to the drain of the NMOS transistor 6, thereby supplying a rectified and filtered voltage. By switching the NMOS transistor 6, a pulse voltage is generated in the secondary winding 7b and the tertiary winding 7c.The pulse voltage generated in the tertiary winding 7c is rectified and filtered by an auxiliary diode 8 and the starting capacitor 4, and its output voltage is supplied to both ends of the power supply control circuit. The pulse voltage generated in the secondary winding 7b is rectified and filtered by an output diode 9 and an output capacitor 10, thereby producing an output voltage.
The operation of the power supply apparatuses as shown in FIG. 1 is described as follows. When an AC voltage is supplied to the power supply apparatuses, the AC voltage is rectified and filtered into a DC voltage by the rectifier 1 and the filter capacitor 2. The DC voltage charges the starting capacitor 4 through the starting resistor. As the voltage rises at point A, the voltage at point B also rises and is supplied to the power supply control circuit 5. When the voltage at point B exceeds a specific voltage level, the power supply control circuit 5 is activated. For example, if the power supply control circuit 5 is a 3842 controller then the voltage at point B must at least equal 16 V to activate the 3842 controller. The power supply control circuit 5 issues a switching signal to switch (turn on and off) the control transistor 6. Then, pulse voltages are generated in the secondary winding 7b and tertiary winding 7c. The pulse voltage generated in the secondary winding 7b is rectified and filtered by an output diode 9 and an output capacitor 10, and thus a DC voltage is supplied to the other device (not shown). The pulse voltage generated in the tertiary winding 7c is rectified and filtered by a auxiliary diode 8 and the starting capacitor 4, and its output voltage is supplied to both ends of the power supply control circuit 5. That is, the power source of the power supply control circuit 5 is supplied through the starting resistor 3 at the outset when AC voltage is applied to the power supply apparatuses. After activating the power supply control circuit, the working voltage for the power supply control circuit is supplied through the tertiary winding 7c of the power transformer 7. However, a current also flows through the starting resistor 3, and electrical power is consumed.
Generally, the input AC voltage specifications of computers and computer peripheral devices are somewhere between 90 V and 264 V. The power loss consumed by the starting resistor can be calculated as follows. Suppose 90 V AC is supplied to the power supply apparatuses, after being rectified and filtered, the DC voltage is about 90 Vxc3x971.414=127.6 V. A 3842 controller is used as a power supply control circuit 5, and the minimal starting current for the 3842 controller is about 1 mA. Therefore, the maximal resistance of the starting resistor can be (127.26 Vxe2x80x9416 V)2 /1 mA)=111.26 K.OMEGA. When the AC voltage equals 264 V, the voltage at point A after being rectified and filtered is about 373.296 V (264 Vxc3x971.414). After the 3842 controller is activated, the voltage at point B drops to about 10 V. Consequently, the power consumption of the starting resistor 3 is about (373.296 Vxe2x80x9410 V)2 /111.26 K.OMEGA.=1.18 W.
In computers and computer peripheral devices, a power-saving function for decreasing the power consumption of these devices when not in the energized state is desired. The power-saving function should decrease the power consumption less than 3 W, and the power loss (1.18 W) in the starting resistor 3 cannot be ignored.
The general power supply circuit can reduce the power dissipation by the starting resistor, however, it still suffers from some problems, while operating with a power-saving IC in a power-saving mode.
In power-saving mode, the power-saving IC will take control of the power supply. The power-saving IC shuts down the unnecessary power output, for example the power output for the horizontal and vertical scanning signals of a monitor, except for a power output with +5 V and 10 mA for the power-saving IC.
To meet the above requirements, one way is to keep the current flows through the starting resistor 3 and the power supply keep operating to allow the power-saving IC to function. In this way, the circuit has a faster power response. However, this increases the circuitry complexity and cost and the power consumption cannot be controlled to satisfy the power requirements.
The object of the present invention is to provide a power supply apparatus, which operates according to the power source signal of a computer host in the normal mode. When the host is in the DPMS mode, the power supply to the monitor is shut down because the host doesn""t output the power source signal. Therefore, the consumption of the monitor will decrease in DPMS mode with reduced circuitry complexity and cost.
To achieve the above-mentioned object, the present invention provides a power supply apparatus for outputting an output voltage according to a first signal from outside, the power supply apparatuses comprise a rectifier, a power transformer, a power control circuit, and a switching device. The rectifier is provided for supplying a DC voltage source. The power transformer receives the DC voltage source, then outputs the output voltage and a second signal. The power control circuit is provided for controlling the output voltage of the power transformer according to the second signal, then the switching device outputs the second signal to the power control circuit according to the first signal.
Moreover, the present invention further comprises a host and a monitor. The host is provided for supplying a first signal, and the monitor comprises a power supply apparatus for supplying an output voltage to the monitor according to the first signal. Wherein the power supply apparatus comprises a rectifier, a power transformer, a power control circuit, and a switching device. The rectifier is provided for supplying a DC voltage source. The power transformer receives the DC voltage source, then outputs the output voltage and a second signal. The power control circuit is provided for controlling the output voltage of the power transformer according to the second signal, then the switching device outputs the second signal to the power control circuit according to the first signal.